A variable compliance device is coupled between a robot arm and a robot tool to provide sufficient compensation to allow for slight misalignment between the robot tool and its environment. When the robot tool encounters a misaligned part or other obstruction, an outside force is applied to the tool. The variable compliance device alows the tool to rotate or to move laterally or longitudinally relative to the robot arm, without damaging the robot arm. When the outside force is removed, the compliance device restores the tool to its original position relative to the robot arm.
If excessive force is applied to a tool, the tool may be moved beyond the limits of the compliance device's ability. U.S. Pat. No. 4,717,003, issued Jan. 5, 1988, to McCormick et al., discloses and describes a variable compliance device that includes a means for sensing movement of a tool attached to the device and for shutting down the robot when an overload condition is encountered. The device can be adjusted by air pressure during operation of the device.
A variable compliance device may also include a quick change adapter, so the robot tool can be easily changed to a different type. A quick change adapter is disclosed and described in U.S. Pat. No. 4,676,142, ssued June 30, 1987, to McCormick et al.
The variable compliance device of the invention includes a tool plate for attachment to a tool and means for coupling the tool plate to a robot arm. The compliance device allows limited longitudinal, lateral, and angular movement of the tool plate relative to the robot arm.
A plurality of bearing balls are located in concave ball seats associated with the tool plate. A pin having a concave surface engages each bearing ball, so that longitudinal, lateral, or angular movement of the tool plate relative to the robot arm caused by an outside force causes at least one of the pins to move relative to the robot arm.
A bias means exerts a force against the pins toward the bearing balls. The force of the pins against the bearing balls resists movement of the tool plate relative to the robot arm. This force also restores the tool plate to its original position relative to the robot arm after the outside force is removed during compliance.
The bias means includes an inflatable diaphragm that can be inflated or deflated to a selected pressure. Inflating the diaphragm to a higher pressure increases the rigidity of the coupling, increasing the force required to move the tool plate relative to the robot arm. The higher pressure also increases the force that restores the tool plate to its original position.
The variable compliance device of the invention also includes a sensor that senses when any of the pins is moved beyond a selected point. The sensor then signals the robot to shut down in order to avoid damage to the robot, the tool, or the workpiece due to an excessive force.